It is known to utilize a rod piston in a well safety valve as shown in U.S. Pat. No. 4,161,219 in order to reduce the hydrostatic well pressure acting on the safety valve and thus allow the valve to be used at greater depths. It is also known, as disclosed in patent application Ser. No. 538,000 now U.S. Pat. No. 4,527,630 entitled Hydraulic Actuating Means for Subsurface Well Safety Valves, to utilize a metal piston and seat for providing positive sealing in environments subject to high temperatures and corrosive conditions. However, it is also important that a safety valve have an outside diameter positioned to fit into a well conduit while at the same time having structural thickness and strength to withstand the pressures encountered, particularly the internal or tubing pressure.
The present invention is directed to an improved subsurface well safety valve which has a small cross-sectional piston area for reducing the hydrostatic pressure and allowing the valve to be set at greater depths, as well as having seal means which will withstand a hostile environment and provide a positive seal, and a valve that has a minimum outside diameter while maintaining adequate structural size to withstand high tubing pressure.